1. Field of the Invention
The present invention relates to a method and apparatus for controlling the application of power to a load. More specifically, the present invention relates to the application of three phase power to a load, such as a motor, by way of a hybrid contactor system.
2. Description of the Prior Art
Conventional motor contactors must be designed to survive the arcing environment of making and breaking inductive loads of a three-phase, 60 hertz system. This arcing at the contacts leads to the use of double-break contact systems to provide sufficient withstand voltage for higher voltage, e.g. 460 V, operation. Also, large amounts of precious metals must be used to withstand the temperatures and the high rates of erosion at the arcing contacts. The high contact gaps and large contact erosion dictate a large air gap and a relatively inefficient magnetic structure with high power consumption required to drive the moving contacts.
A further problem is the constraint on system design due to the mechanical size and shape of the conventional electromagnetic contactor. The present electromechanical contactor is constrained to roughly a cubic shape where none of the dimensions are more than twice any other dimension. For a reversing contactor, two of these fully rated contactors must be used with a mechanical and/or electrical interlock to prevent short circuit conditions during the reversal of the motor.
To avoid the problems associated with conventional contactors, the use of hybrid contactors for providing loads with large amounts of current, is common. Generally, such hybrid contactors use a semiconductor device in parallel to a pair of relay contacts. The semiconductor device avoids or minimizes any arcing which would otherwise occur in the relay contacts. The arcing might otherwise cause degradation of the relay contacts and, eventually, necessitate the replacement of the relay. Moreover, the use of the semiconductor devices minimizes transient effects on the load caused by the bouncing of the relay contacts on either the make or break of a circuit and such hybrid contactor systems are shown for example by U.S. Pat. Nos. 3,466,503, issued to Goldberg; 3,446,991, issued to Howell; 3,504,233, issued to Hurtle; and 3,736,466, issued to Fox et al, all assigned to the assignee of the present invention. Another such hybrid contactor system is disclosed in the co-pending and commonly assigned application for "Relay Switching Apparatus" by Charles W. Eichelberger, Ser. No. 069,618, filed on Aug. 27, 1979 now U.S. Pat. No. 4,296,449 issued Oct. 20, 1981.
The use of commutated contacts is well documented in the literature. In general, the prior art relies upon the establishment of a voltage across the protective contacts before a three-terminal solid-state device is turned on to commutate the arc which is established at those contacts. A second area of prior art includes the basic concept of diode commutation of two series-connected relays as shown, for example, in the above-mentioned Goldberg patent and the above-mentioned Eichelberger patent application. Although the prior art diode commutation systems have been useful, they have generally required a large number of relays to provide both a complete ohmic break (i.e., on all 3 lines) between source and load and a reversing function.